This invention relates to firing dental porcelain on metal in manufacturing dental prostheses and in particular to the cool-down procedure.
The procedure for firing dental porcelain to fuse the dental porcelain with the metal in manufacturing dental prostheses producing good aesthetic and performance characteristics has been known for a long time. The bonding system for fusing the dental porcelain to metal begins essentially with the application of one or more layers of unfused dental porcelain material to a metal support or substrate. The dental porcelain is conventionally applied in several layers of varying compositions and shades. Usually the initial layers are dental opaquing porcelains, and then there is a middle dental body porcelain and/or stains and then a covering enamel. Each individual layer is usually fired and appropriate tooth anatomy is carved in each layer. Therefore, it will be understood that the dental porcelain materials are fused on the substrate which may be a crown or bridge substructure made of a special precious metal alloy, a semi-precious metal alloy, or a non-precious alloy by means of several successive firing processes. The firing temperature for each layer's successive firing is usually between 900.degree. C. and 980.degree. C. The firing is usually carried out under a temporary partial vacuum in a firing chamber. The final firing is generally referred to as a glaze firing.
The thermal expansion of the bonding alloys during multiple heating and cooling phases between room temperature and approximately 980.degree. C. is usually relatively constant. However, the highly feldspathic materials of the usual dental porcelains which are fused to the metal can have considerable variation in performance because of e.g. the crystallization in the glass ceramic system K.sub.2 O.Al.sub.2 O.sub.3.SiO.sub.2 by forming leucite. The problem is specifically noted here with regard to these highly feldspathic materials which are present in the preferred dental porcelains. It is understood that in the temperature range of approximately 600.degree. to 900.degree. C. crystallization occurs, and dependent on this crystallization, an increase of heat expansion results, dependent on temperature and time.
The repetitious heating and/or cooling modes of the various build-up procedures tend to cause compressive or tensile stresses which can lead to chippings, fissures, or cracks at the porcelain interfaces with the metal or alloy. This can result in an unsatisfactory product necessitating a remake.
The achievement of desirable low compressive stress in dental porcelain/metal bonding systems is additionally complicated by the fact that the variety of alloys has increased considerably in recent years. The coefficient of thermal expansion can vary between 138 to 154.times.10.sup.-7 /K. (K.=Kelvin) over the temperature interval of 25.degree. to 600.degree. C. customarily measured. Therefore, the danger of strong compressive or tensile stresses in dental porcelains fused to metal in such bonding systems is increased and failures caused by cracks or chipping of the veneering layers are increasingly likely.
During the cooling phase of the firing procedure used to manufacture a dental porcelain fused to metal prostheses, there arises a temperature gradient in progression from the dental porcelain's face or outer surface inwardly to the metal. This temperature gradient is higher when rapid cooling takes place. This means that especially in the dental porcelain solidification temperature range, the metal normally shows a lower temperature than the dental porcelain veneer. The stresses in the fused cold dental porcelain to metal interface appear to be substantially directly proportional to this difference in temperature.
It is an object of the present invention to provide a porcelain metal prostheses that will meet clinical test and provide a long, useful prostheses life.
It is another object of the present invention to provide a dental porcelain/metal prostheses having no inherent tensions that would produce cracks or chips.
It is a further object of the present invention that at room temperature the dental porcelain is under essentially low compressive stress.
A still further object of the present invention is to improve the manufacturing procedure for producing dental porcelain veneer prostheses to eliminate or ameliorate the danger of cracking or chipping, especially when a variety of highly feldspathic materials are to be used with a wide array of different alloys.